The present invention is drrected generally to circuit condition indicating devices, and more particularly to a capacitively coupled cable-mounted indicating device having improved cable engaging and coupling means for use in conjunction with a high voltage cable.
It is frequently necessary to monitor one or more specific circuit conditions on the conductor of a high voltage cable to determine whether certain condition exist, such as the absence of voltage or the presence of a fault current. The high voltage present on the conductor requires that the indicator be suspended directly on the conductor without communicating power or signal lines, or other metallic connections which might present a shock hazard to linemen or others involved in the servicing of the cable. This necessitates that the indicator either be battery powered, or be powered directly from the conductor by means of electrostatic coupling to the electric field surrounding the conductor, or inductive coupling to the magnetic field accompanying current flow through the conductor. The electrostatic coupling method is most appropriate for monitoring high voltage power system cables, and it is to the efficient accomplishment of this method that the present invention is directed.
The use of electrostatic coupling to the conductor of a high voltage power transmission system requires that the indicator be positioned close to the conductor so that a sufficient and consistent coupling is obtained with the the electric field surrounding the conductor. A further requirement of such coupling is that the indicator be readily installable and removable on the cable, preferably by means of a conventional lineman's tool, so as to not add to the inherent shock hazard of high voltage systems. The monitoring device of the present invention provides an improved construction which satisfies these requirements.
To realize the potential gradient within the electric field surrounding the conductor necessary for powering the device circuitry prior cable-suspended circuit condition monitoring devices utilized one or more electrically conductive projecting members. Unfortunately, such projecting members provide a potential leakage path on the outer surface of the device housing, between the exposed member and the monitored cable, which acted as a shunt impedance to the circuitry of the device which could under certain circumstances prevent the device from receiving sufficient excitation from the electric field surrounding the conductor. The construction of the present invention completely avoids the need for such projecting appendages, thereby achieving a more reliable and more compact monitoring device for use on high voltage power distribution systems.
The construction of the monitoring device of the invention is particularly well adapted for detecting the occurrence of fault currents in a monitored high voltage conductor, as shown in the copending application of the present inventor entitled "Fault Indicator Having Improved Trip Inhibit Circuit", filed concurrently herewith, and will be shown in that context herein. However, it will be appreciated that the construction of the monitoring device is adaptable to other purposes, such as a loss of voltage indicator, as shown in the copending applcation of the present inventor entitled "Voltage Loss Detector", filed concurrently herewith, and also for providing indications of voltage levels in a conductor.
Accordingly, it is a general object of the present invention to provide a new and improved circuit condition monitoring device for mounting on the high voltage conductor of a power distribution system.
It is a more specific object of the present invention to provide a circuit condition indicating device having a improved cable mounting arrangement whereby the indicator can be readily installed on and removed from a monitored conductor.
It is another specific object of the present invention to provide a circuit condition indicating device having an improved electrostatic coupling arrangement for developing operating power from a conductor from which the device is suspended without the use of projecting electrically-conductive ground plane elements.